While sometimes lengths of elastic cable or "shock cord" are used to support trampoline beds (see U.S. Pat. No. 2,916,746, for instance), far more prevalent are extensible metal springs having integrally formed hooks at their ends. Typically, these are all of uniform, overall free length, at least initially, and are helically wound with their coils stacked on each other. Indeed, often the coils are also wound with a certain amount of "backload," as it were. This is done to provide an increased initial tension and thus shorten the distance the spring must be pulled out, as compared with a "non-backloaded" spring, to bring it up to the desired tension for the bed. Hence the bed can be larger relative to the frame with "backloaded" springs than it can with "non-backloaded" springs. Nevertheless, after a while all springs, even "backloaded" ones, tend to become overstretched, particularly after periods of hard use or by heavy performers, and thus lessen the tension on the bed. Several springs can become severely overstretched should they be accidentally stepped on by a performer. Sometimes even the springs are overstretched when being initially installed. Once overstretched the springs customarily have to be replaced. There are other problems, too.
If the frame or the bed is not exactly rectangular, or if either is of an odd side, obviously the tension on the bed will not be uniform, or will be all too much or all too little, since the springs are all initially of equal free length. Nor can the tension of the bed be adjusted to accommodate the weight or skill of performers. Some of the heavier performers or those who bounce extremely high may be in danger of striking the floor or even the trampoline undercarriage, while lighter or less skillful performers may prefer a less "hot" bed, as those with greater tension or resiliency are sometimes called. Furthermore, when a trampoline is folded up, as can be done with most by folding the third or so of the frame and bed at each end over onto the middle third (see U.S. Pat. No. 3,116,809, for example), the tension on the springs across each end of the bed is so reduced that they can and often do become detached from the bed or even fall off completely. The hooks at the ends of the springs, since they are integrally formed from the same relatively small diameter wire that makes up the bodies of the springs, tend to wear the spring anchors along the trampoline frame rather rapidly so that those anchors must be repaired or the frame replaced. In short, then, the simple, integral metal springs so long used for trampolines are in fact innately deficient in quite a number of respects which, depending upon the conditions, impede the adaptability and performance of the trampolines of which they are so vital a part.
For years, for some reason, these impediments in trampolines have nevertheless been unwittingly suffered and only recently have received any really serious attention. It was then recognized that the source of the foregoing deficiencies lies primarily in the springs themselves. This in turn resulted in a first step which ultimately lead to the present invention. That first step was to make the area of the bed somewhat smaller and then employ one or more "spring extenders" on each spring. These extenders are simply bent from heavy wire and include an "eye" at one end and a hook at the other, the distance between the two being about one inch. Hence, when the springs are initially installed, one or two such extenders are used at the end of each spring adjacent the frame to provide the proper initial spring tension. Then when the springs become overstretched, instead of discarding them, proper tension on the bed can be restored simply by removing one or both of the extenders. This is possible owing also to the recognition that, on account of Hooke's Law and within limits of course, an overstretched spring retains the same elastic characteristics as it had initially. That is to say, for example, that if the free length of a "non-backloaded" spring is initially nine inches and to provide proper tension it has to be pulled out to thirteen inches, then if it later becomes overstretched so that its free length is, say eleven inches, it will still provide the same tension if it is pulled out to fifteen inches. If the spring was also initially "backloaded," however, it might have to be pulled out only to twelve inches in order to provide the same tension as the "non-backloaded" spring when pulled out to thirteen inches. But once a "backloaded" spring becomes overstretched, since the "backload" is thereby destroyed, it too might have to be pulled out to fifteen inches in order to provide the same tension as the overstretched "non-backloaded" spring. Consequently, the distance "backloaded" springs must be pulled out to provide proper tension once they are overstretched is significantly greater than before because then, in effect, they have the same characteristics as "non-backloaded" springs.
While the spring extenders allow overstretched springs to be reused, and also permit adjustment of bed tension, they are not wholly satisfactory for a number of reasons. In the first place, they permit adjustment only in fixed intervals, i.e., the length of one extender. In the second place, the total number of them for a single trampoline, several hundreds in fact, means a great number of separate, relatively small parts which require extra time and care to install and which easily become lost or misplaced. When "backloaded" springs are used, even more extenders are initially necessary in order to allow sufficient distance for adjustment should they become overstretched since, as pointed out above, the springs then have the characteristics of "non-backloaded" springs. Spring extenders also exacerbate the problem of loose springs across the ends of the bed when a trampoline is folded up because many of the extenders invariably fall off on the floor and are lost in this fashion. Nevertheless, they did prove out the recognition that it is the springs themselves, especially their fixed free length, which are the source of the many deficiencies which plague the adaptability and performance of current trampolines.